1. Field of the Invention
The present invention relates to an inkjet recording ink, ink cartridge, and inkjet recording method.
2. Description of the Related Art
In recent years, as an image formation method, an inkjet recording method has become popular because the inkjet recording method provides a simpler process than other recording methods, full color is easy to make, and high-resolution images can be obtained even by a simple device.
According to the inkjet recording method, a small amount of ink is jetted by foam generated by heat, or by pressure that is generated with the use of a piezoelectric or electrostatic force, or the like. The ink that has adhered to a recording medium such as paper is promptly dried, or penetrates into the recording medium. In this manner, an image is formed. The use of the inkjet recording method has become widespread in various fields, from personal and industrial printers to printing.
For an inkjet recording device, as a colorant, a dye ink that uses a water-soluble dye is mainly used. However, the disadvantage is that the dye ink is poor in weather resistance and water resistance. Therefore, in recent years, instead of the water-soluble dye, research on a pigment ink that uses pigment has been underway. However, the pigment ink is still poorer in color-producing performance, ink jetting stability, and storage stability than the dye ink.
As the high-quality image technology of OA printers has advanced, the pigment ink is increasingly required to have the same level of image density as the dye ink even when plain paper is used as a recording medium. However, when plain paper is used as a recording medium, the problem with the pigment ink is that the ink penetrates into the paper, resulting in a decrease in the pigment concentration of the paper surface and a drop in image density.
In recent years, in particular, demand for industrial applications has been growing, and high-speed printing has been desired. For high-speed printing, an inkjet printer with a line head is proposed. In the case of high-speed printing, in order to increase the drying speed of the ink that adheres to the recording medium, such a printer has a unit configured to speed up the drying process by adding a penetrating agent to the ink and allowing water to penetrate into the recording medium. However, at this time, the degree of penetration of pigment into the recording medium, as well as of water, becomes higher. As a result, the image density further decreases.
To improve image density, various methods are proposed. For example, the following ink is proposed (for example, see Japanese Patent Application Laid-Open (JP-A) No. 2006-16412): the ink is so formed that the viscosity-rise rate (mPa·s/%) associated with moisture evaporation of the ink is less than 5.0 mPa·s/% when the amount of moisture evaporation relative to the total weight of the ink is less than 30% by mass, and that, when the amount of moisture evaporation is between 30% by mass to 45% by mass, the viscosity-rise rate is greater than 50 mPa·s/%.
Even when the proposed ink is used for high-speed printing, the penetration of pigment into the paper is suppressed, because there is a rapid increase in the viscosity of the ink as moisture evaporates after the ink landed on the plain paper. As a result, the image density is said to be high. However, in the case of the proposed ink, there is a rapid increase in the viscosity as moisture evaporates. Therefore, if moisture evaporates in an inkjet recording device, particularly in a nozzle, an increase in viscosity causes problems, such as a reduction in ink jetting stability, or the fixing of ink around the nozzle.
Moreover, the following ink is proposed (for example, see Japanese Patent (JP-B) No. 3155318): the ink contains polymer gel microspheres that become higher in viscosity due to swelling as proton concentration moves to the acidic side. That is, when the ink is stored or is placed in a recording device, the ink is so adjusted as to have an alkaline-side proton concentration. However, after the ink landed on the plain paper, the proton concentration moves to the acidic side as protons are supplied from the recording medium, or plain paper. As a result, the polymer gel microspheres become higher in viscosity, and the penetration of pigment into the paper is said to be suppressed. According to the proposal, even if moisture evaporates in a nozzle of an inkjet recording device, there is no increase in viscosity, and the ink jetting stability does not become lower. However, the image density is insufficient, and there is still room for further improvement.